1. Field of the Invention
The present invention relates to a dielectric filter for use in microwave bands and the like, a dielectric duplexer, and a communication apparatus using them.
2. Description of the Related Art
Conventional filters for use in microwave bands and the like include a dielectric filter that is formed by providing an inner-conductor-forming opening or inner-conductor-forming openings in a dielectric block to form one resonator or a plurality of resonators, and by providing an outer conductor on outer faces of the dielectric block.
In the conventional dielectric filter using the dielectric block, terminal electrodes coupled by electrostatic capacitance to the inner conductors are provided to perform unbalanced-type input and/or output of signals. Therefore, to feed signals to, for example, a balanced-input/output-type amplifier circuit, a balun (balanced-unbalanced converter) is needed to convert unbalanced-type signals to balanced-type signals. However, the dielectric filter of this type gives rise to problems. Insertion loss because of the balun is large. Also, spacing must be secured for arranging the balun on a circuit substrate, and miniaturization cannot therefore be implemented.
To solve the above-described problems, Japanese Unexamined Patent Application Publication No. 7-254807 discloses a dielectric filter capable of performing balanced-type input and/or output of signals. In this balanced-type dielectric filter, however, to electrically connect an inner conductor to terminal electrodes on an outer face of a dielectric block, openings must be formed perpendicular to an inner-conductor-forming opening. It is very difficult to form the openings perpendicular to the inner-conductor-forming opening in the dielectric block. Therefore, manufacturing costs are increased. In addition, since both balancing characteristics and the degree of external coupling are influenced by the positions of the openings that form the aforementioned connecting conductors, high forming accuracy is required to obtain predetermined characteristics. Also, adjustment after forming is difficult.
The present invention provides a dielectric filter, a dielectric duplexer, and a communication apparatus using them, which allow balanced-type input and/or output of signals to be implemented, and concurrently, allow manufacturing costs to be easily reduced.
According to one aspect of the present invention, a dielectric filter is made by forming a conductor film on a single dielectric material to provide at least two xcex/2 resonators that individually have two ends being open-circuited, that resonate at xc2xd-wavelength at a predetermined frequency, and that are coupled together. Terminals are individually coupled by electrostatic capacitance to vicinities of the two open-circuit ends of each of the xcex/2 resonators and are used for balanced input and/or output of signals.
According to the above aspect of the invention, without a balun being used, direct connection can be made to a balanced-input/output-type amplifier circuit, since balanced terminals are used to perform input and/or output of signals. Concurrently, either passage or attenuation of a predetermined frequency band can be performed. Therefore, the dielectric filter can be miniaturized, and production costs therefor can be reduced. In this case, since the resonators and the balanced terminals are coupled by the electrostatic capacitance, filter characteristics can easily be adjusted.
Also, according to the above aspect of the invention, each of the two xcex/2 resonators may either be formed using a microstrip line or a strip line, or be formed of a dielectric coaxial resonator made by providing the conductor films in and on a dielectric block. In this case, no balun needs to be formed, and a filter for performing balanced input and/or input of signals can be easily formed on a dielectric substrate.
In addition, each of the two xcex/2 resonators may be formed of a dielectric coaxial resonator made by providing the conductor films in and on a dielectric block. In this case, despite the dielectric filter being formed of the coaxial resonators, mounting of only the dielectric filter on a mounting substrate or the like allows a circuit for performing balanced input and/or output of signals and a circuit having the filter to be concurrently formed. Therefore, the area occupied on the mounting substrate can be reduced, and the mounting efficiency can be improved.
Furthermore, according to another aspect of the present invention, a dielectric duplexer is formed by providing a transmission filter and a reception filter that have the above-described configuration. In this case, the dielectric duplexer can be used as an antenna-sharing unit that performs balanced input and/or output of transmission signals and/or reception signals. In addition, miniaturization can be implemented for amplifier circuits and the like that perform balanced input and/or output as well as a high-frequency circuit section in the vicinity of the antenna.
According to still another aspect of the present invention, a communication apparatus is formed by using one of the above-described dielectric filters and/or the above-described dielectric duplexer. Therefore, a small and light communication apparatus can be constructed.
Other features and advantages of the present invention will become apparent from the following description of embodiments of the invention which refers to the accompanying drawings, in which like references indicate like elements and parts.